[codex] Rename Python SDK AppServerConfig to CodexConfig (#24800)

## Why

`AppServerConfig` is exported as part of the ergonomic Python SDK
surface and passed to `Codex(...)` and `AsyncCodex(...)`. That name
exposes the underlying app-server transport at the same layer where
users are configuring the Codex client. `CodexConfig` makes the common
callsite read naturally and names the object it configures.

## What changed

- Renamed the public configuration dataclass from `AppServerConfig` to
`CodexConfig`.
- Updated `Codex`, `AsyncCodex`, and the transport clients to accept
`CodexConfig`.
- Updated binary-resolution messages, package exports, docs, examples,
and related coverage to use the new public name.

## API impact

```python
from openai_codex import Codex, CodexConfig

with Codex(config=CodexConfig(codex_bin="/path/to/codex")) as codex:
    ...
```

Callers should now import and construct `CodexConfig`; `AppServerConfig`
is no longer part of the Python SDK surface.

## Validation

- `uv run --frozen --extra dev ruff check src/openai_codex scripts
examples tests`
- Tests are deferred to online CI for this PR.

sdk/python/src/openai_codex/__init__.py

@@ -22,10 +22,10 @@ from .api import (
     TurnHandle,
     TurnResult,
 )
-from .client import AppServerConfig
+from .client import CodexConfig
 from .errors import (
-    AppServerError,
-    AppServerRpcError,
+    CodexError,
+    CodexRpcError,
     InternalRpcError,
     InvalidParamsError,
     InvalidRequestError,
@@ -41,7 +41,7 @@ from .retry import retry_on_overload
 
 __all__ = [
     "__version__",
-    "AppServerConfig",
+    "CodexConfig",
     "Codex",
     "AsyncCodex",
     "ApprovalMode",
@@ -64,10 +64,10 @@ __all__ = [
     "SkillInput",
     "MentionInput",
     "retry_on_overload",
-    "AppServerError",
+    "CodexError",
     "TransportClosedError",
     "JsonRpcError",
-    "AppServerRpcError",
+    "CodexRpcError",
     "ParseError",
     "InvalidRequestError",
     "MethodNotFoundError",